The present invention relates to a device for supplying power to an external data medium reader unit connected to a computer, in particular a microcomputer, from a port, in particular a PCMC-A port or a parallel port. It is intended more particularly for portable computers. The invention also relates to a method of supplying power used in this device, and to an external reader unit including this device.
A great difficulty in the adaptation of a hard disk and/or a CD-ROM drive to the PCMCIA port of a portable computer results from the low powers available on the power supply pins of PCMCIA ports. In order to supply these peripherals without recourse to an external power supply, two voltages are available: a general power supply voltage Vcc, equal to 5 V or to 3.3 V, and a secondary power supply voltage Vpp, equal to 12 V.
Furthermore, in the case of an external reader unit connected to a parallel port of a computer, it is in any case necessary to provide another access to the first power supply voltage source (Vcc), no power supply voltage generally being available on this type of port.
In practice, a maximum current of the order of 500 mA can be taken from the general 5 V power supply, but this current is often limited by an internal protection resistance of the order of 1 .OMEGA.. The result of this is that, when this current is taken, the output voltage is reduced to below the threshold of TTL logic: 4.75 V, which does not allow a direct supply of logic circuits of power greater than 1 W.
The maximum current permitted to be taken from the secondary 12 V power supply is most often of the order of 60 mA, with a typical internal resistance of 2 .OMEGA..
Furthermore, these power supplies are very often protected by an internal circuit breaker whose result is either that the power supplies of the PCMCIA port are cut off, or that the computer is reinitialized as soon as the current demanded exceeds the maximum permitted value.
Now, with the increase in the performance of external reader units, and in particular in the speed of rotation of the read heads, peak current requirements become increasingly higher and the present power supply devices are not capable of meeting them.